“Living polymerization” is a form of addition polymerization in which chain termination reactions are absent or are strongly inhibited. Polymer chains typically grow at a more constant rate in living polymerization than is seen in other types of polymerization, and the resulting chain lengths tend to be similar (i.e., the polymers have a very low polydispersity index). Living polymerization is useful for synthesizing block copolymers. A block copolymer can be synthesized in two or more stages, with each stage containing a different monomer.
One method that has been used for preparing PTs and their block copolymers is living polymerization based on the Ni-catalyzed Kumada polymerization of 5-bromo-2-thienylmagnesium monomers. Mechanistic studies have suggested that this polymerization proceeds through a cyclic catalysis mechanism involving a series of oxidative addition/reductive elimination steps. To take full advantage of the “living” polymerization mechanism, the reaction is typically conducted at ambient temperature. The reaction generally yields polymers having low to medium molecular weights due to the low reactivity of the Ni(II) catalysts that have been used, such as Ni(dppp)Cl2 (where dppp is 1,3-bis(diphenylphosphino)propane). The Ni center typically has a square-planar geometry. However, in this system it is possible for the propagating Ni(II) reactive center to transfer from one chain to another during polymerization, so this polymerization might more accurately be described as “quasi-living.” True “living” polymerization would be enhanced by the availability of highly reactive universal catalytic systems.
Prior approaches to preparing regioregular, high molecular weight poly(3-alkylthiophene)s have required relatively high temperatures and long reaction times. As a consequence, the resulting polymers have shown decreased regioregularity (85-95%), and despite high reaction temperatures and long reaction times, have still shown relatively low molecular weights. The difficulty in obtaining high-regioregularity (around 100%) polythiophenes in substantial amounts hinders development of practical applications of these materials and has prompted some studies of less-regioregular polythiophenes as possible substitutes.
V. Senkovskyy et al., J. Am. Chem. Soc. 2007, 129, 6626-6632; R. Tkachov et al., J. Am. Chem. Soc. 2010, 132, 7803-7810; and H. Bronstein et al., J. Am. Chem. Soc. 2009, 131, 12894-12895 reported an externally initiated living polymerization process in which a stable aryl-Ni(II) initiating complex, e.g. σ-complex 1 (FIG. 1A), was used to catalyze the polymerization of Grignard monomer 2. The catalytic initiator was stabilized by a bidentate phosphine ligand. The initiator was prepared by ligand exchange between the initial complex with monodentate phosphine ligands which, in turn, may be prepared by oxidative addition of an aryl halide to Ni(PPh3)4 (FIG. 1A). This external initiation route enhances the utility of living catalyst-transfer polymerization. For example, it makes it possible to grow surface-immobilized PT brushes. However, the previously-reported process requires a relatively complicated preparation to make the catalytic initiator; and it is possible for the initiator to be contaminated with monodentate PPh3 ligand. Any such contamination can decrease catalytic activity, and limits practical applications of this method.
C. Amatore et al., Organometallics 1988, 7, 2203-2214 reported that Ni(0) complexes containing bidentate phosphine ligands do not react with aryl halides.
T. Yokozawa et al., Chem. Rev. 2009, 109, 5595-5619 provides a review summarizing recent research in chain-growth condensation polymerization.
N. Marshall et al., Chem. Commun., 2011, 47, 5681-5689 provides a review summarizing recent research in surface-initiated polymerization of conjugated polymers.
M. Iovu et al., Macromolecules 2005, 38, 8649-8656 reported a Grignard metathesis polymerization of 3-alkylthiophenes by a quasi-living chain growth mechanism using a 1,3-bis(diphenylphosphino)propane]dichloronickel(II) (Ni(dppp)Cl2) initiator. The authors reported that the reaction proceeded through a cycle of oxidative addition/reductive elimination steps.
R. Miyakoshi et al., J. Am. Chem. Soc. 2005, 127, 17542-17547 hypothesized that the mechanism for chain-growth polymerization of 2-bromo-5-chloromagnesio-3-hexylthiophene with Ni(dppp)Cl2 involved a coupling reaction between a Grignard thiophene and the growing polymer via the Ni catalyst, which was transferred intramolecularly to the terminal C—Br bond of the elongated molecule, a mechanism that the authors called a catalyst-transfer polycondensation. That is, the polycondensation was said to proceed with the catalyst transferring to and activating the elongated polymer end group.
E. Lanni et al., J. Am. Chem. Soc. 2009, 131, 16573-16579 proposed mechanisms for the Ni(dppe)Cl2-catalyzed chain-growth polymerization of 4-bromo-2,5-bis(hexyloxy)phenylmagnesium chloride and 5-bromo-4-hexylthiophen-2-ylmagnesium chloride. Both polymerizations exhibited first-order dependence on the catalyst concentration, but were nearly independent of the monomer concentration. 31P NMR spectroscopic studies suggested that the resting states were unsymmetrical NiII-biaryl and NiII-bithiophene complexes. In combination, the data suggested reductive elimination was the rate-determining step for both monomers, followed by subsequent intracomplex oxidative addition, leading to chain growth.
V. Senkovskyy et al., J. Am. Chem. Soc. 2007, 129, 6626-6632 described a method to grow conductive polymer brushes of regioregular head-to-tail poly(3-alkylthiophenes) via surface-initiated, catalyst-transfer chain growth polycondensation of 2-bromo-5-chloromagnesio-3-alkylthiophene. The method used a Ni(II) macroinitiator formed by reaction of Ni(PPh3)4 with photo-crosslinked poly-4-bromostyrene films. Exposing the initiator layers to the monomer solution led to selective chain growth polycondensation of the monomer onto the surface, thereby producing conductive polymer brushes. The brushes were said to be mechanically robust, and to be stable against delamination.
H. Bronstein et al., J. Am. Chem. Soc. 2009, 131, 12894-12895 reported the polymerization of a thiophene Grignard reagent, initiated from an externally added cis-chloroaryl(dppp) nickel complex, to produce a regioregular poly(3-hexylthiophene) with controlled molecular weights and narrow polydispersities.
Regioregularity has often been considered to be an important factor in optimizing the electronic properties of polythiophenes. See, e.g., A. Carella et al., “Synthesis and application to OPV of highly regioregular polyalkoxyphenylthiophenes catalyzed by copper complexes,” Joint Italian-Israeli Workshop on Organic PV, Portici, Italy, Oct. 20, 2011. However, there have also been reports that the regioregularity of the polymer may not have a significant effect on the power conversion efficiency of a photovoltaic device. See, e.g., R. Mauer et al., Adv. Funct. Mater. 2010, 20, 2085-2092.